Film stretching device



Sept 27, 1955 M. o. LoNGs-rRE-I'H ET AL 2,718,658

FILM STRETCHING DEVICE 2 Sheets-Sheet 1 Filed Oct. 5, 1955 INVENTORS. Mar/ey O. Longs/re 2% BY Dan/'e/ /4/. Ryan TTORA/EY Sept. 27, 1955 M. o. LONGSTRETH ET A1. 2,718,658

FILM STRETCHING DEVICE Filed Ooi. 5, 1953 2 Sheets-Sheet 2 ,@MMMMMMMMMM MMIMI ,47' 7' ORNE KS United States Patent FILMi DEVICE Murray-f 0. Longstreth r lVIidland,l lvlich., and: Daniely W. Ryan, Palos Verdes Estates, assignors to The Dow Chemical., (20.11.11.any,` Midland,` Michi a corporation of Delaware Application October 19.53, Serial No.,3,84',1125, 4 Claims. (,Cl. ltr-,1)

invention relates to an; improved, device for the stretching, or orienting of foils.y and film, and. particularly to means, for assuringY uniformity o f gauge of hiaxially,y oriented polystyrene. film.,

Fihns and foils made of` certain thermoplastic:- polymers. are. highly useful. For example, as, described in U. S., Patent 2,074,285, the, ilmsmade from polystyrene are very valuable as. an. insulating tape for wrapping electrical cables. Unplasticized polystyrene in its usual state is very brittle, but it is known that when the molecules of lqiolystyrenev arel arranged or oriented in a certain manner iny thinl films, the films. are fiexiblev and can be readily wound. around cables and the like. To: accomplish this. orientation, it is necessaryI to stretch,l the material biaxially` during, prOdllGtiQn. 0f the film, Various. devices, have been. made for attaining the desired orientation. One of these has been reported by Horn in Kunststoffe., 30p. 53 (1940).` In. the method described by Horn, a polystyrene tube i's, expressed through a. circular die and over a divergent spreader mandrel' which has a plurality of parallelA rollers along its periphery to aid inrminimizingfthe friction between, the mandrel and the, sliding film. A pair of rolls is located beyond the fr iandrell vand perpendicular tothe direction. of travel of the film. By operating these rolls at a` peripheral speed. greater than the speed of the plastic through the die, a simultaneous longitudinal stretch combined with the transversestretch applied bythe mandrei providestheproper:biaxi-al orientation 0f the polystyrene molecules.. to attain the desired flexible film. However, when it is attempted to make Wide sheets of thin fil-mtl vmil or less). usi-ng the above apparatus, many difficulties are encountered. Not the least4 important of these is a variationl in the, gauge. of the film across its width, To be useful for commercial purposes, a film must have a'uniform thickness.

' ItA is an object of' this invention to provide an improved film stretching device for imparting a biaxial orientation to a thermoplastic film. l

It is a further object to provide an improved film stretching device for producing thermoplastic films having a uniform gauge.

It has now been found that thermoplastic films of uniform gauge may be produced by means of a modification of the previously described machine in which heat reflecting plates are mounted in a plane parallel to the plane surfaces of the film forming mandrel, and at a distance not to exceed 2 inches from the path of the stretched film, in position to reflect heat radiated from the film back to such areas of the film surface as tend to be too thick. Such strategically located refiecting plates equalize the plasticity of the film, by proper heat distribution across its face, and allow the film to be stretched uniformly.

Fig. l is a front elevation of the machine, with the refiecting plate shown in outline;

Fig. 2 is a fragmentary cross-sectional view along line 2 2 of Fig. 1;

Fig. 3 is a fragmentary view of a refiector plate as- ICC 2. sembly, seen from the outer side. disposed away from the mandrel; and

Fig. 4 is a similar fragmentary vievl ofthe reflector plate assembly of Fig. 3, seen from, the inner, side.l

For ease of understandinghere, willffijrst be, described the prior apparatus to be improved.

The machine of which thisv inyention an improved modification consists of a means for expressing; a. thermo.- pl'astic material through a circular die 10 or other means of Supplying an unoriented tubular. film,l and. a. divergentspreader mandrel 11 aligned withand attached toy the core of the die 1,0. When` so attached, spreader mandrel 11 will be completely within thet'ube of` material. rifhe useful divergent edges of the vmandrel 1 1, may be straight or convex, spreadingy o ut symmetrically from the end at which the filmv is fedY onto the spreader mandrel 11. Concave edges do. no t give the desired results.

Disposed. along the outer edges and on both sides of thev mandrel. 11 from a point near die. 1Q are freely rotating, rollers 12y which facilitatey conveying ythe tube. past the outer edges of the mandrel 11A without frictional drag. It is preferred that the rollers- 1,2 on one face of the mandrel 11 be in staggered relationship tothe rollers 12` on the other face of the mandrel 11,. Rollers 1,2` may be mounted on mandrel 11 by means of peened stub Shafts 24 (Fis- 2.)- Sueh anarransement provides maximum support for the hoty movingy film.L

A pair of driven rolls 13 is located' beyond the` spreader mandrel 11 and extends the width of thel mandrel 11,k in position to engage the stretched' and flattened tube. By operating the rolls 1 3 at aperipheral speed greater than the speed at which the polymer is',v being extruded, the rolls provide longitudinal stretch to the film at the same time as the radial stretch imparted by the mandrel' 11.

It is preferred to cool the hot film issuing from the die 10 at the points where the film will contact the rollers 112 to prevent it from sticking thereto. A suitable means for cooling the film isa plurality of air jets 14j mounted a short distance away from and in the plane of spreader mandrel 11, so that air streams mayl bey directed at the tube offpolymer as it moves over` the edge of the mandrel 11. To supplement the cooling effects of the air jets 14, it is preferredA to provide a waterj'acketed mandrel. An especially suitable form of such a mandrel is described in our" copending application, Serial No. 327,363, filed December 22', 19.52-, now Patent 697,248. In this preferred embodiment water jackets 15j are installedl in the edges of the. mandrel 11 nearest die 10. Water intake 16 and outlet 17 tubes are connected to jackets 1,15 so that continuouscirculation of a heat transfer liquid is possible.

Since it is desired to cool the film only at the edges of mandrel 11, it is necessary to insulate the other parts of mandrel 11. A suitable means of accomplishing this insulation is described in our copending application, Serial No. 327,363, filed December 22, 1952. In the embodiment described in that application, a layer 18 of insulating material, such as glass wool, covers mandrel 11, extending at least as low as the lower edge of the water jacket 1S, and as wide as the distance between the rollers 12. A smooth cover sheet 19, of stainless steel or other suitable material, holds the insulating material in position.

The tube of stretched polymer is commonly slit into two flat sheets. Suitable means for slitting the tube are described in our copending application, Serial No. 327,362, filed December 22, 1952, and comprise sharp edged cutting devices 21, such as razor blades, mounted in brackets 22 rigidly attached to spreader mandrel 11.

In the improvement of this invention one or more polished plates is mounted adjacent the mandrel 11 to serve as heat reflective surfaces, as mentioned above. 1t is preferred that `the reector plates be adjustable, to make it unnecessary to remove and remount them each time a new distribution of heat is encountered in the film. A satisfactory and preferred embodiment of an adjustable heat-reflective unit is shown in Figs. 3 and 4. In'that embodiment, a rigid frame 30 has equally spaced holes 31 drilled vertically through its upper and lower edges. Rotatably tted in holes 31 are shafts 32, which may have knurled knobs 33 at their outer ends and which may have slots 34 at their inner ends for -receiving and holding narrow rectangular polished plates 35. Each ofthe many parallel plates 35 may be rotated individually to any position so that its face is disposed at any angle from to 180 with respect to the plane of the film stretching mandrel. Heat radiated from any portion of the lm may be allowed to pass between fully opened plates, or it may be reflected toward either side by partially opened plates, or it may be reected back toward its source by a closed plate. These adjustments may be made as required without changing the mounting of the reflector assembly.

Plates 36 may be aixed, as by screws 37, to the top and bottom edges of the inner side of frame 30 for the dual purpose of vincreasing the reecting area and shielding the knurled knobs 33.

It should be understood that the plate illustrated represents only an embodiment of a suitable apparatus and that other forms of the apparatus are possible. It is desirable that the reliector assembly, when mounted, not extend beyond the edge of the mandrel, to prevent overheating the portion of the tubular lm which must contact the rollers 12.

For ease in mounting the reflector plates in position, brackets 38 may be attached to frame 30 with bolts 39. The brackets 38 may then be attached to any suitable rigid support lfor holding the plates in position.

Frame 30 should be mounted parallel to the mandrel 11 and less than 2 inches away from the mandrel 11, preferably about 1/z inch. For best results, the frame 30 should be mounted from 12 to 18 inches from the die 10. When located further from the die, the lm has time to'cool which lessens the effectiveness of the reector assembly.

p In operation, ,a thermoplastic material, such as polystyrene, is expressed through a die so as to form a tube of hot stretchable material. The tube then moves downward and over the spreader mandrel 11 which causes the tube to be transversely stretched. At the point where the tube comes into contact with the mandrel 11, an externallyl mounted air jet 14 directs a stream of air over that part of the tube which will contact the edges of the mandrel 11. This part of the lm is cooled further by conduction from the cooling jackets 15. The

film moves over the edges of the mandrel 11 on freelyrotating rollers 12 which minimize the friction' ofthe film sliding over the edges. The lm is stretched longitudinally by a pair of driven rolls 13 extending across the width of the mandrel 11. The tube is slit by razor blades so that two at sheets are obtained. The plates 35 of this invention are mounted opposite the points on the mandrel 11 wherever thick sections of the film may occur. The knobs 33 may be turned so that the amount of heat dissipated from the film may be controlled evenly across the width of lms. j

When polystyrene lm was prepared 'using the apparatus of this invention the maximum variation in gauge across the width of the lm was 0.0003 inch.

By way of contrast, when a similar film was prepared without the reecting plates the observed variation in gauge was 0.0013 inch.

We claim:

l. In apparatus in which a molten thermoplastic is extruded downwardly from an extrusion orifice in tubular form and stretched about a hat spreader having two parallel plane faces to form thin film, the improvement which consists in adjustable heat reflective elements disposed parallel to and not over 2 inches from a plane face of the lilm stretcher, each such retiector having a plurality of vertically disposed, individually adjustable metal vanes having smooth reective surfaces, and means for rotating each such vane about its vertical axis to position its face at any angle from 0 to 180 with respect to the plane face of the i'llm spreader to direct reection of radiated heat from the adjacent hot lm surface as required to adjust the thermoplasticity of the lm to a substantially uniform value in the film stretching area.

2. Apparatus as claimed in claim l, wherein a set of the adjustable retective elements is disposed on each side of and parallel to both plane faces of the lrn spreader. 3. Apparatus as claimed in claim 1, wherein the heat reective element is positioned about 1/2 inch from the said plane of stretched film.

4. Apparatus as claimed inclaim l, wherein the heat reective element is positioned about 12 to 18 inches below the extrusion orifice.

References Cited in the le of this patent UNITED STATES PATENTS 2,318,203 Crosby May 4, 1943 2,318,533 Selvig May '4, 1943 FOREIGN PATENTS 45,266 France July 22, 1935k 655,014 Germany Aug. 7, 1936 

1. IN APPARATUS IN WHICH A MOLTEN THERMOPLASTIC IS EXTRUDED DOWNWARDLY FROM AN EXTRUSION ORIFICE IN TUBULAR FORM AND STRETCHED ABOUT A FLAT SPREADER HAVING TWO PARALLEL PLANE FACES TO FORM THIN FILM, THE IMPROVEMENT WHICH CONSISTS IN ADJUSTABLE HEAT REFLECTIVE ELEMENTS DISPOSED PARALLEL TO AND NOT OVER 2 INCHES FROM A PLANE FACE OF THE FILM STRETCHER, EACH SUCH REFLECTOR HAVING A PLURALITY OF VERTICALLY DISPOSED, INDIVIDUALLY ADJUSTABLE METAL VANES HAVING SMOOTH REFLECTIVE SURFACES , AND MEANS FOR ROTATING EACH SUCH VANE ABOUT ITS VERTICAL AXIS TO POSITION ITS FACE AT ANY ANGLE FROM 0* TO 180* WITH RESPECT TO THE PLANE FACE OF THE FILM SPREADER TO DIRECT REFLECTION OF RADIATED HEAT FROM THE ADJACENT HOT FILM SURFACE AS REQUIRED TO ADJUST THE THERMOPLASTICITY OF THE FILM TO A SUBSTANTIALLY UNIFORM VALUE IN THE FILM STRETCHING AREA. 